The present invention relates to a semiconductor device and, more particularly, to a bipolar semiconductor device which enables an increase in integration density.
A typical conventional bipolar semiconductor device has heretofore been arranged such that a heavily-doped buried layer providing a lead for a collector is formed in a surface region of a silicon substrate and an island which is made of single crystal silicon is formed above the buried layer, as described in the specification of Japanese Pat. Laid-Open No. 56-1556 (1981). In this prior art, an active region which is necessary for transistor operation is formed in one single crystal silicon island. Another island (sub-collector region) made of single crystal silicon is provided above the buried layer, in addition to the single crystal silicon island, having the active region formed therein in such a manner that the sub-collector region is connected to the buried layer. A collector electrode is led out from the surface of the silicon substrate through the sub-collector region.
Furthermore, isolation between the emitter and base regions of the conventional transistor is effected using an insulating film formed on the surface of the base lead-out electrode. The emitter and base electrodes are disposed so that they do not planarly overlap each other.
The above-described prior art suffers, however, from the problem that, as transistor sizes shrink, the area ratio of the active region, necessary for a transistor operation to the sub-collector region becomes substantially 1:1. Therefore, it is therefore difficult to further reduce the area required to provide transistors.
In addition, since the emitter and base electrodes must be disposed so as not to planarly overlap each other and spaced apart from each other by a predetermined distance, the prior art has difficulty in increasing integration density.